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From seabirds to Arctic mammals, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Oral Clues

It is not the first time scientists have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have found humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Probably they were kissing," the researcher noted, explaining that the concept chimed with studies that has revealed people of non-African ancestry contain ancient genetic material in their genome, demonstrating interbreeding was occurring.

Romantic Interpretation

"It certainly puts a more romantic spin on ancient interactions," the lead researcher said.

Publishing in the journal Evolution and Human Behavior, Brindle and her team detail how, to investigate the historical roots of kissing, they first had to develop a definition that was not restricted by how people smooch.

Describing Intimate Contact

"There have been some efforts to describe a intimate act, but it's very much been focused on humans, which implies that basically non-human species don't kiss. Currently we know that they probably do, it may appear different from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she said some actions that resembled kissing were distinct activities – such as the processing and food sharing, or "kiss-fighting", seen in fish known as certain marine animals.

Consequently the team developed a description of kissing centered around friendly interactions involving directed oral interaction with a individual of the identical group, with some movement of the oral area but absence of nutrition.

Study Approach

Brindle said they concentrated on accounts of intimate behavior in non-human species from the African continent and Asian regions, including primates, apes and orangutans, and used online videos to confirm the observations.

The researchers then integrated this information with information on the genetic connections between extant and ancient species of such primates.

Evolutionary Timeline

Researchers propose the results indicate kissing developed approximately 21.5m and 16.9m years ago in the ancestors of the large apes.

Placement of Neanderthals on this family tree suggests it is likely they, too, indulged in a kiss, the researchers conclude. But the behavior may not have been limited to their own species.

"Reality that modern people engage intimately, the reality that we now have shown that ancient relatives probably engaged, indicates that the both groups are probably did engage," Brindle noted.

Biological Importance

While the scientific reasoning is discussed, Brindle explained intimate contact could be used in sexual contexts to potentially increase mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of primates said that as intimate contact was observed in a wide range of primates it was logical its roots lie deep in our ancient history, and an examination of various types of intimate behavior among a wider variety of animals might push its origins back even earlier still.

"Things that we think of as characteristics of our species, like kissing, are not unique to us if we examine carefully at other animals," he said.

Cultural Aspects

An archaeology expert said that kissing had a cultural element as it was not universal to all societies.

"However, as people we thrive or fail on the strength of our relationships, and ways of promoting trust and closeness will have been significant for millions of years," she said. "It might be an image that seems a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it should be expected that Neanderthals – and even them and our own species together – kissed."